2003 ST 4 Stroke 9.9/15HP Johnson outboards Service Manual, Page 154Get this manual
POWER UNIT CRANKCASE VENTILATION SYSTEM
CRANKCASE VENTILATION SYSTEM
The blow-by vapor which is generated inside the crankcase is separated into the liquid and vapor components inside the space which is bounded by the head cover and the head cover gasketOnly the resulting vapor component is sent via the breather hose to the silencerAfter this, it is mixed with fresh air, and the resulting mixture is drawn into the combustion chamber to improve combustion efficiency